Coherency

[nikita-klsh]

Introduce:

• Different coherency measures for semblance calculation, e.g. gather.calculate_semblance(mode=mode)
• Stretch muter(compensate the effect of nmo correction): Muter.from_stacking_velocity MF.from_refractor_velocity_field
• Crossover muter(compensate the effect of events time reversal after nmo correction): gather.apply_nmo(crossover_mute=True)
• Linearly segmented colorbar for semblance plot, configured by 2 parameters: semblance.plot(clip_threshold_quantile=0.99, n_levels=10)
• gather.mute() now use default value fill_value=np.nan. This forces the explicit presence of np.nan in gather.data.
  The majority of actions works fine, not all tho, e.g. gather.mute(muter).resample(kind='cubic') currently not properly work.